The present invention relates to thermal ink jet printing, and in particular, to a composition, substrate, method and apparatus for obtaining secure thermal ink jet printing.
The desktop ink jet printer using thermal ink jet technology, otherwise known as Bubblejet.TM. (a trademark of Canon) or OfficeJet.TM. (a trademark of Hewlett Packard) printing, has become one of the most commonly used printing machines in the office environment. A thermal ink jet printer is a variation on the ink jet printer concept that uses heating elements instead of piezoelectric crystals to shoot ink from nozzles. When the printer is used, it is typically driven by a personal computer.
This is an age of data and information explosion, and the volume of information communicated through different techniques grows on a continuous basis. However, among all the different media used, information transmission via printing remains one of the most important means of communication.
The need to process an enormous volume of printed information has naturally dramatically raised the concern for the security of such information. The concept of security in this context can be classified into two broad types, the ability to seal such information such that one can be assured that only the receiver will be able to unseal and read the information and, to protect the integrity of printed information which cannot be sealed in a tamper-proof manner such that any fraudulent attempt to alter some vital printed information, such as a name, a number, etc., can be easily detected and ideally to enable the original information to be retrieved.
There is currently a great need for a convenient way of achieving printed information security when a document is being printed, when it is being copied or when it is being faxed. U.S. Pat. Nos. 5,424,266, 5,476,830 and 5,532,200 have disclosed methods of rendering printed documents secure by means of latent images obtained by impact printing or by thermal printing, whereby the information remains invisible and hence virtually sealed until revealed through the use of a special highlighter that performs the action equivalent to the opening of the sealed envelope.
Although the known methods are efficient in a certain number of situations, it has been discovered that a dramatically more efficient and convenient device and method based upon the utilization of thermal ink jet printing technology will provide not only a new and more convenient way of rendering printed documents secure by means of latent image printing, but, provide many additional elements of flexibility whereby, for example, originally visible printed information can be checked to see if it is the legitimate original information, and in addition, when the printed information is tampered with in certain situations, it allows one to retrieve the original information.
Conventional thermal ink jet printers have ink reservoirs in the form of ink cartridges which can be refilled or replaced when empty. Single color ink jet printers usually have a single ink jet cartridge which typically stores black ink. Color thermal ink jet printers either use a three cartridge system including cyan, magenta and yellow inks or a four cartridge system which uses cyan, magenta, yellow and black ink cartridges.
The text stored in the computer, or any other text processing equipment memory, is then fed to the printer which prints it in a single color or in any one of the colors which can be achieved with the colored inks.
The printer can be driven by a computer to command the printer to print different parts of the text in different desired colors using conventional printer drivers. Thermal ink jet printing has also been used in high quality color copiers where the color information picked up from a color original by a scanning head of the color copier is appropriately fed to the color printer head of a thermal ink jet printer equipped with a three or four color ink cartridges. Moreover, printing by way of plain paper fax machines have been developed using thermal ink jet technology. The digitized information scanned from a document in a fax machine is used to drive electronic circuitry of the thermal ink jet printhead to produce a fax copy.
The thermal ink jet printing process is a relatively recent development, and U.S. Pat. No. 4,723,129 is one of the earliest dealing with the thermal ink jet method to produce projected droplets of ink for printing on a substrate. The system involves projecting an ink of a mostly aqueous composition from a small nozzle in the form of droplets formed by the instantaneous pressure built up within the ink holding container. Different methods have been disclosed to be used as mechanisms for the instantaneous pressure buildup causing the projection of droplets of ink on the recording paper substrate and thereby effecting printing.
The ink used in a thermal ink jet printing system is expected to meet certain basic requirements: it must be free from clogging problems in the nozzle, it must maintain stable physical properties when in storage without producing any precipitate and it must insure a sharp contrast upon recording. The physical properties must also have magnitudes which fall within certain required ranges, and these relate to viscosity, surface tension, thermal expansion and thermal conductivity. It is generally recognized that the tolerance ranges for the required values of these latter parameters in a thermal ink jet ink are quite wide and that the extreme values of the ranges are about an order of magnitude apart. Since water happens to exhibit physical property values that fall within the required ranges, mostly aqueous inks, where the ink additives do not dramatically disturb the above mentioned properties, would be generally expected to provide functional thermal ink jet ink systems provided such additives also allow the ink to respect the previously mentioned basic, nozzle clogging prevention and stability requirements.
Another consideration used in the formulation of aqueous inks is related to the prevention of the buildup of bacteria. This is achieved through the addition of a minute amount of antiseptic, approximately 0.1% by weight of Dioxine is generally used for this purpose.